The present invention relates to an evaluation of a flat panel display, more specifically, relates to a technique for evaluating a non-uniformity on a flat panel display with consideration for an observation angle with respect to the display.
A display device referred to as a so-called flat panel display such as a liquid crystal display, an organic electroluminescent display, and a plasma display is required to provide uniform display all over the display screen. Since display devices are affected by manufacturing variations in quality of each component, not all of the display devices are capable of providing uniform display properties in the whole range of the display area. For this reason, in a display device inspection, display devices are made to operate and are inspected to find out whether or not display non-uniformity (hereinafter also called mura or a mura defect) occurs in their display areas.
FIG. 10 shows mura (non-uniformity) detection images generated by a conventionally known mura evaluation method. As shown in FIG. 10, four mura defects are observed in the display area of a display device. Each mura defect is marked with an evaluated value indicating a degree of mura defect. Here, the higher the evaluated value, the more noticeable the mura defect.
In the conventional method, mura defects are detected by changing an inspection angle, as shown in FIG. 10. When a mura defect is detected from different angles, the mura defect is determined as having different evaluated values indicating the degrees of mura at the respective angles, and as having an area with different shapes and sizes. The conventional method, however, has a problem that a mura distribution in each observation direction cannot be intuitively determined. Moreover, because the mura defects in the observation direction cannot be intuitively determined, the conventional method further has problems that the conventional method fails to provide an evaluation result equivalent to that in a sensory test, and to make information on a mura distribution inside each mura defect available for effective use.
Various kinds of methods have been heretofore proposed for mura evaluation of a display device. For example, Japanese Patent Application Publication No. 2008-64629 discloses a method including: capturing images of a display, which is to be inspected, under a condition at multiple levels; acquiring multiple first images from the image-capturing results; generating multiple second images by processing these first images so that a change in each of the first images can be emphasized; generating a composite image by synthesizing these second images with predetermined weights assigned to the second images; and determining the presence of a mura defect on the basis of the composite image. In JP-A 2008-64629, the assigned weights are determined by using multiple second images generated from a display for reference having a mura defect. More specifically, the weights are determined so that an area having the mura defect can be identified distinctively from the other area in a composite image generated by synthesizing the second images of the display for reference.
Japanese Patent Application Publication No. 2007-174355 discloses an evaluation apparatus that evaluates a display by excluding a luminance change along with a change in an observation (visual) angle. This evaluation apparatus includes: an image capture that captures multiple images of respective parts of a display at different imaging positions; means for performing interpolation computation on the image information of the imaged parts of the display whose images are captured at the respective imaging positions by the image capture; and means for generating an luminance image showing the luminance of the display measured in a certain direction and for quantifying mura from the luminance image.
Japanese Patent Application Publication No. 2007-93466 discloses an evaluation assisting apparatus for an evaluation of a color image display device including a screen in which pixels are arranged in a two dimensional array, the pixels each providing a display color that is viewed as different colors in different observation directions. This evaluation assisting apparatus evaluates color reproduction properties of the color image display device when the observation angle is changed, in a method including: figuring out an observation direction of each of the pixels when the screen displaying a color image for evaluation is viewed from a first position; figuring out the viewed color of the color image when the pixel is viewed in the figured-out observation direction from the first position; figuring out an observation direction of each of the pixels when the screen displaying the color image is viewed from a second position; figuring out the viewed color of the color image when the pixel is viewed in the figured-out observation direction from the second position; obtaining a color difference between the viewed colors of each of the pixels figured out when the screen is viewed from the first and second positions, respectively; and calculating an evaluated value of the display device from the color difference of each of the pixels.
Meanwhile, as a method of detecting mura in a display area of a display device, a method of automating a sensory inspection for measuring display mura of a display device is also disclosed by the present inventors in Nakano and Mori, “Quantitative Approach of Sensory Evaluation By Using Machine Vision Technique,” Journal of the Institute of Electrical Engineers of Japan, Vol. 125, No. 11, pp. 696-698 (2005).
In addition, a method of qualitatively measuring mura by using an SEMU method and correcting the mura through a wavelet transform is disclosed in Mori and Nakano, “Quantitative evaluation method of liquid crystal displays' mura (luminance nonuniformity) by using wavelet transform,” Papers of Technical Meeting on Information Oriented Industrial System, Vol. IIS-05; No. 11-20, (D) Industry Applications, The Institute of Electrical Engineers of Japan.
In the method described in JP-A 2008-64629, multiple first images are captured by changing the shooting angle or the like; second images having their contrasts changed from those of the first images are generated from the first images, and then are synthesized to generate a composite image; and then whether or not a mura defect is present is determined based on the generated composite image. Thus, JP-A 2008-64629 discloses the method in which a result similar to a result of a sensory test conducted by an inspector is generated using the multiple images captured at the different angles. This method, however, only allows a determination as to the presence of mura by performing image processing on images captured at different angles and then synthesizing the images. In other words, even with use of this method, the angular distribution and size of mura cannot be determined through evaluation.
In JP-A 2007-174355, the display colors of the display device are observed to detect how each of the display colors is changed in color tone depending on the observation direction, and thereby mura is evaluated as the distribution of color reproducibility at the observation angle. The use of this method allows mura to be evaluated from the viewpoint of hue change that is one of compound factors for generating mura. However, a mura defect also occurs depending on the luminance distribution as well as the hue distribution. For this reason, even with use of the method of JP-A 2007-174355, it is not possible to evaluate how a mura defect of a display device is changed in absolute size depending on the observation angle.
Moreover, the method of JP-A 2007-93466 is also only to synthesize first images to generate a second image for the purpose of evaluating mura in the color reproduction properties. In the generation of the second image, the first images are weighted depending on their shooting angles. In this way, the method of JP-A 2007-93466 is not a method of providing values equivalent to the values resulting from a sensory test (organoleptic test), to evaluate how a mura defect is changed in absolute size along with a change of the observation angle, and to evaluate the absolute size of the mura defect.
As described above, mura of a display device is changed in luminance and color tone along with a change in the observation angle. When image processing such as image synthesis is performed to evaluate the mura thus changed, information on a range where the mura occurs or other similar information may be obtained from the synthesized image. Such method, however, can neither quantify mura itself with consideration for the observation angle so that the quantification result of the mura can indicate a physiological intensity in a recognizable manner equivalent to that of a sensory test, nor display the quantification result as if the result of the sensory test is displayed.
Mura of display devices is considered important as one of quality requirements common to panel production sites, makers and users. In addition, the range of viewing angle is also considered important as one of display quality requirements. For this reason, in terms of quality control and for the purpose of improvement of inspection efficiency, quantitative measurement of mura has been demanded to obtain an indicator, equivalent to that of the sensory test, indicating how mura exists in the display area within the range of the viewing angle to be supported by a display device, and how the mura looks depending on the observation angle.
In summary, there has been heretofore demanded a technique that allows mura in the display area of a flat panel display device to be graphically displayed and to be intuitively determined by quantifying, as an indicator equivalent to a result of a sensory test, the presence of the mura and the intensity of the mura on the human vision that even changes depending on the observation angle.